1. Field of the Invention
The present invention relates to an easy-to-use single-photon generation apparatus for generating a single photon, and a quantum bit reading apparatus and method.
2. Description of the Related Art
A single-photon source is an important apparatus for quantum information processing techniques, such as quantum computers utilizing a photon, or quantum encryption techniques, and is required to eject a single photon at desired timing in a particular spatial mode.
There is a method for most simply and reliably satisfying the requirement, in which in principle, a π-pulse (π: circular constant) laser is used to excite a two-state system, such as a single atom, ion, molecule or quantum dot, and a photon of a frequency corresponding to a resonator mode is ejected in the resonator mode as a result of the coupling of the two-state system and the resonator mode (with coupling constant g), and is further ejected at a dissipation constant κ (>g) in a particular spatial mode set outside the resonator. In this method, a period of π/Ω is necessary for excitation, and a period of π/g is necessary for photon ejection. Immediately after photon ejection, the two-state system is restored to its initial state, and next photon ejection cycle can be started. Since Ω=2πE·μ/h (E: Laser electric field; μ: Transition dipole moment), if a strong laser beam is used, the cycle (interval) of generation of a single photon, i.e., π/Ω+π/g, becomes π/g, which is the highest operation speed for a single-photon source utilizing a resonator mode. In this method, however, the angular frequency (wavelength) of strongly excited light is equal to an angular frequency (wavelength) corresponding to the resonator mode, and the light becomes stray light (noise) and is liable to mix with a single photon to be used.
To avoid this problem, it is considered to use a single three-state system, in which a transition corresponding to excitation is made different from a transition corresponding to photon ejection by the coupling of the system with the resonator mode. Further, there is another method using adiabatic passage without using excitation to an upper state, although it employs a three-state system of a single physical system.
However, in these methods, at least a period of approx. 1/g is required until a photon is generated after the initial state (e.g., |1>), and immediately after generation of the photon, the state is not restored to the initial state (for example, it is kept at |2>). Therefore, it is necessary to wait until the initial state (|1>) is restored by a spontaneous transition between |2>−|1> that is generally longer than the other two transitions, or to restore the state to the initial state using adiabatic passage that is caused by application of light with two wavelengths. Accordingly, in both cases, the repetition frequency is reduced by a degree corresponding to the time required to restore the state to the initial state. In addition, in the latter case, it is necessary to apply light of two wavelengths and control the intensity of the light. Also in this case, light of the same frequency as that of the resonator mode is applied, which inevitably causes the same problem as in the case of using the two-state system.
There is also a method in which a three-state system is contained in a resonator that has a mode resonating with a transition between |2>−|1>, and has a dissipation constant greater than the coupling constant of the mode and transition between |2>−|1>, and the transition between |2>−|1> is accelerated by the coupling of the mode and transition between |2>−|1> to quickly restore the initial state (see, for example, Japanese Patent No. 3682266). In this method, the time required for the restoration process can be reduced, but it is necessary to employ anther resonator. Further, it is desirable that no restoration process be needed between different energy states.
Furthermore, a method of generating a single photon in a microwave area has been recently developed, in which the transition frequency of a physical system considered as a two-state system in a superconductive state, called Cooper pairs box, is changed to cross a resonator mode to eject a microwave photon in the resonator mode (see, for example, American Physical Society March Meeting 2007, Publication No. H33-5).
Yet further, there is a method in which a two-state system is excited using adiabatic passage caused by laser beam application, and is used as a single-photon source. In this method, however, photons are ejected in various modes (directions), and no method of ejecting photons in a particular mode is disclosed.
A simple method of ejecting a single photon is not known, which differs from the conventional methods requiring a lot of time, or being complex and requiring a restoration process wherein stray light may be intensified, and which can realize a repetition frequency of approx. g/π, and in which excitation light differs in frequency (wavelength) from a resonator mode, i.e., a photon ejected.